A study about partial discharge measurements performed applying to insulating systems square voltages with different rise times

Partial Discharge (PD) measurements output, when non-sinusoidal voltages are applied to insulating systems, can have different meanings, depending on the voltage waveform frequency content and on the PD probe circuit. A PD detection circuit is designed for a low noise-to-signal ratio, thus for suppression of the residual high frequency (HF) voltage. Such a suppression varies with the applied voltage rise time: for fast rise rate voltage pulses the PD measurements deserve a specific study. When multiple PD sites are active in the same insulation system and when they give rise to PD inception during the voltage rise/fall, effects due to the signal superimposition should be considered. During quasi-square wave PD tests on twisted pairs the voltage rise rates, the voltage test levels and the high PD repetition rate during voltage "pulses" have been suitably inter-played, taking in account analogue and digital PD acquisition systems, in order to evidence the PD measurements actual information content. The obtained results evidenced the limits of high and low frequency PD measurements; in certain instances they did negate both the validity of the relation <PD apparent charge vs signal amplitude> and the possibility of resolving each single PD pulse.